In recent years, photoacoustic tomography, in which an optical characteristic value distribution of an organism interior is determined in high definition using a characteristic of an acoustic wave (typically an ultrasound wave) that produces less scattering in the organism interior than light has been proposed. In this specification, photoacoustic tomography is referred to as PAT. When an organism is irradiated with pulsed light generated by a light source, the pulsed light propagates through the organism interior while diffusing. Photoabsorbers contained in body tissue absorb energy from the propagated pulsed light and generate an acoustic wave. By subjecting a resulting acoustic wave signal to analysis processing, an optical characteristic distribution of the organism interior, and in particular an optical energy absorption density distribution, can be obtained.
A photoacoustic mammography (to be referred to in this specification as PAM) apparatus has been proposed as an apparatus for applying PAT to an organism. A PAM apparatus detects the position of a breast tumor by forming an image of new blood vessels formed on the periphery of the tumor during tumorigenesis and areas having a high absorption coefficient and including the new blood vessels. To diagnose an entire breast, it is important not only to obtain a high definition image, but also to shorten a diagnosis time by widening a measurement area per measurement operation.
A PAM apparatus is mainly constituted by an illumination optical system serving as an input system and an ultrasound wave detection system serving as an output system. Various constitutions have been proposed for the illumination optical system up to the present day. In a rear detection type photoacoustic apparatus in which the illumination optical system and the ultrasound wave detection system are disposed on an identical side, a dark field illumination optical system for observing a superficial site of an organism surface has been proposed as a photoacoustic wave microscope (see U.S. Patent Application Specification No. 2006/0184042, hereinafter called PTL 1). An illumination optical system that inputs light beams from both sides of a linear array ultrasound probe has also been proposed (see Joel J. Niederhauser, Michael Jaeger, Robert Lemor, Peter Weber, and Martin Frenz, IEEE TRANSACTIONS ON MEDICAL IMAGING, vol. 24, no. 4, 436, hereinafter called NPL 1).